refactor(svelte-query): rename FunctionedParams<T> to Accessor<T> for improved semantics and consistency, use in function return types

[braden-w]

This change renames FunctionedParams<T> to Accessor<T> to improve code clarity and align with established patterns in the ecosystem. The change brings several key benefits:

1. Alignment with Established Standards: The name Accessor<T> aligns with established patterns, particularly SolidJS's Accessor utility type.

For example, you can already see this same pattern is used in solid-query's createQuery.ts, createQueries.ts, isRestoring.ts

2. Semantic Clarity: The term "Accessor" better describes the type's purpose - a function that provides access to a value. The previous name FunctionedParams was ambiguous and could be misinterpreted as parameters that have been "functioned" or transformed.

3. Removal of Redundancy: The word "Params" in FunctionedParams<T> was redundant since this type is already used in parameter positions. For example:

export function createQuery<T>(
  options: Accessor<CreateQueryOptions<T>>,  // Clearer than FunctionedParams
  queryClient?: QueryClient
)

4. Broader Type Utility: The Accessor type can be consistently reused to type function return types across the codebase, particularly in utility hooks like useIsFetching, useIsMutating, useIsRestoring, and context functions. This creates a unified type pattern for functions that provide access to reactive values.